The invention relates to a method of recording binary information in a magneto-optical storage layer by heating a selected area of the storage layer to approximately the Curie temperature by means of a laser beam of is deviation in time for every bit, and to an arrangement for carrying out the method.
Magneto-optical storage layers are made of a ferrimagnetic material deposited on a glass or plastics substrate, generally together with further dielectric layers. The storage layer is premagnetised in a direction perpendicular to the plane of the layer and the direction of magnetisation can be reversed by means of an external magnetic field. The strength of this magnetic field can be small if the material of the storage layer has been heated to approximately the Curie temperature.
For the storage of binary information in an area of the magneto-optical storage layer this area is heated to approximately the Curie temperature by means of a focussed laser beam, and the magnetisation in the heated volume of the storage layer can be reversed by means of an external static magnetic field to form a magnetic domain. The information is read via the Kerr effect, which produces a change in the state of polarisation of the light reflected from the disc depending upon the direction of magnetisation of the storage layer. The binary value of the information can then be detected by determining the direction of polarisation of the light reflected from the storage layer.
A method of the type defined in the opening paragraph for a rotating storage layer is known, for example from "Philips Technical Review", Vol 42, No. 2, August 1985, pp.37 to 47. In this method the binary information is converted into channel bits in such a way that at least three bits of equal binary value succeed one another, as is also apparent from the reference cited therein "Philips Technical Review", Vol. 40, No. 6, 1982, pp. 157 to 164. Each bit is recorded in the storage layer by means of a laser beam of limited duration and the bits follow each other so closely that the areas of incidence of the associated laser beams for adjacent bits partly overlap one another. In this way coherent magnetic domains of different length are formed in the storage layer.
The reliability with which a stored information value can be read depends on the signal-to-noise ratio, which should be as large as possible. This signal-to-noise ratio briefly referred to as SNR, depends inter alia on the conditions during information recording, in particular on the energy or energy density of the focussed laser beam at the location where the information is to be recorded, i.e. the area to be heated. Experiments conducted by the Applicant have shown that the SNR value increases as the radiation energy increases, but rapidly tends to assume a limit value. The rise to the limit value proceeds more rapidly and the limit value is reached more rapidly as the pulse by means of which a specific laser energy is applied to the area to be heated of the storage layer is shorter. In particular on account of the limited laser power the lower pulse-duration limits must not be surpassed, because a minimum energy is required for adequately heating the volume at the location of the area to be inscribed.